Methods of fracturing subterranean zones with less pumping

ABSTRACT

Methods of fracturing subterranean zones with less pumping are provided. The methods basically comprise the steps of providing an aqueous fracturing fluid comprised of a brine having a density in the range of from about 9 to about 19 pounds per gallon, pumping the aqueous fracturing fluid into the subterranean zone at a rate and pressure sufficient to fracture the subterranean zone and recovering the aqueous fracturing fluid from the subterranean zone.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to methods of fracturing subterranean zones penetrated by well bores with reduced wellhead pressures and hydraulic horsepower requirements.

[0003] 2. Description of the Prior Art

[0004] Hydraulic fracturing production stimulation treatments are commonly utilized in subterranean hydrocarbon producing zones penetrated by well bores. In such treatments a treating fluid, referred to in the art as a fracturing fluid, is pumped through the well bore into a subterranean zone to be treated at a rate and pressure such that fractures are formed and extended into the subterranean zone. The fracturing fluid carries particulate proppant material, e.g., sand into the fractures which functions to prevent the fractures from closing. That is, the proppant material is deposited in the fractures when the fracturing fluid is broken and recovered. As a result, the formed fractures are prevented from closing whereby conductive channels are formed through which produced fluids can flow to the well bore.

[0005] In order to pump a heretofore used fracturing fluid at a rate and pressure sufficient to fracture a subterranean zone, special elaborate pumping equipment in addition to the pumping equipment used for drilling and cementing the well bore must generally be utilized. The use of such pumping equipment is time consuming and expensive to mobilize, set up and use.

[0006] Thus, there are needs for improved methods of fracturing subterranean zones which utilize the pumping equipment available at the well site or at least reduce the additional pumping equipment required.

SUMMARY OF THE INVENTION

[0007] The present invention provides improved methods of fracturing subterranean zones penetrated by well bores which meet the needs described above and overcome the deficiencies of the prior art. The methods basically comprise the following steps. An aqueous fracturing fluid comprising a brine having a density in the range of from about 9 to about 19 pounds per gallon is provided. The fracturing fluid is pumped into the subterranean zone at a rate and pressure sufficient to fracture the subterranean zone and the fracturing fluid is recovered from the subterranean zone.

[0008] The methods of this invention eliminate the need for pumping equipment in addition to the pumping equipment already at the well site or substantially reduce the extra pumping equipment required as a result of the aqueous fracturing fluid utilized having a very high density. The pumping equipment that is generally associated with a drilling rig is known in the art as “mud pumps.” Off-shore rigs also include additional pumping equipment known as the “cementing unit.” Mud pumps are high rate pumps that are usually limited to wellhead pressures in the range of from about 2000 to about 7000 psi. The high density of the fracturing fluid of this invention produces a bottom hole pressure sufficient to fracture a subterranean zone with considerably lower wellhead pressure and hydraulic horsepower being required than when conventional lower density fracturing fluids are utilized.

[0009] The objects, features and advantages of the present invention will be readily apparent to those skilled in the art upon a reading of the description of preferred embodiments which follows.

DESCRIPTION OF PREFERRED EMBODIMENTS

[0010] The present invention provides improved methods of fracturing a subterranean zone penetrated by a well bore which utilize lower wellhead pressures and reduced hydraulic horsepower requirements. The methods basically comprise the following steps. An aqueous fracturing fluid comprising brine having a density in the range of from about 9 to about 19 pounds per gallon is provided. The aqueous fracturing fluid is pumped into the subterranean zone at a rate and pressure sufficient to fracture the subterranean zone and the aqueous fluid is then recovered from the subterranean zone.

[0011] Because the fracturing fluid has a density considerably higher than prior art fracturing fluids, lower wellhead pressures and reduced hydraulic horsepower are required. As a result, fracturing procedures can be carried out in accordance with the methods of this invention utilizing the pumping equipment available at the well site or utilizing less auxiliary equipment than otherwise would be required. As is understood by those skilled in the art, the well head pressure equals the bottom hole treating pressure minus the hydrostatic pressure plus the friction pressure.

[0012] In addition to utilizing a high density brine fracturing fluid to lower wellhead pressures and reduce the hydraulic horsepower required, a larger work string through which the high density brine fracturing fluid is pumped can be utilized to further lower pressures and reduce horsepower. For example, instead of a 3″ inside diameter work string, a 4″ inside diameter work string or larger can be utilized.

[0013] Also, most mud pumps are incapable of pumping a fracturing fluid containing proppant material, e.g., graded sand for propping the fractures open. In order to prevent the use of additional pumping equipment on off-shore rigs, the rig cementing unit can be used to pump a high proppant material concentration slurry and the mud pumps can be used to pump the high density brine fracturing fluid with the two streams being metered together.

[0014] The brines which can be used in accordance with this invention and that have densities in the range of from about 9 to about 19 pounds per gallon include, but are not limited to, calcium chloride brine, calcium bromide brine, sodium bromide brine, sodium chloride brine, potassium formate brine, cesium formate brine, zinc chloride brine, zinc bromide brine, and mixtures thereof. Of these, calcium chloride brine is preferred.

[0015] In order to increase the viscosity of the brine and thereby form a viscous aqueous fracturing fluid, a gelling agent can optionally be combined with the brine. The gelling agents which can be utilized include, but are not limited to, hydroxyethylcellulose, guar, xanthan and succinoglycan. Of these, xanthan is preferred. When used, the gelling agent is included in the aqueous fracturing fluid in a general amount in the range of from about 0.1% to about 2% by weight of the brine in the aqueous fracturing fluid, most preferably in an amount of about 0.5%.

[0016] In some fracturing applications wherein a higher viscosity fracturing fluid is required than that produced by a gelling agent alone, a cross-linking agent can be included in the fracturing fluid. The cross-linking agent cross-links the gelling agent in the fracturing fluid which has the effect of substantially increasing the viscosity of the fracturing fluid. Examples of cross-linking agents which can be utilized include, but are not limited to, sodium borate decahydrate, zirconium oxychloride, calcium salts, aluminum salts, magnesium salts, iron compounds, iodine compounds, and boron compounds. Of these, sodium borate decahydrate is preferred. When used, the cross-linking agent is included in the aqueous fracturing fluid in an amount in the range of from about 0.1% to about 66% by weight of the gelling agent in the aqueous fracturing fluid, more preferably in an amount of about 1.5%.

[0017] In fracturing operations wherein proppant material is placed in the created fractures, the proppant material is usually suspended in a viscosified end portion of the fracturing fluid. After placement of the fracturing fluid containing the proppant material, the fracturing fluid is broken whereby it reverts to a thin fluid and the proppant material is deposited in the fractures. The proppant material functions to prevent the formed fractures from closing whereby conductive channels are formed through which produced fluids can flow to the well bore. Examples of proppant material which can be used include, but are not limited to, graded sand, sintered bauxite, walnut hulls, and glass beads. Of these, graded sand is preferred. The proppant material utilized is generally present in the aqueous fracturing fluid in an amount in the range of from about 1 to about 18 pounds per gallon of the fracturing fluid.

[0018] All or part of the proppant material in the fracturing fluid can be coated with a delayed hardenable resin composition whereby after the proppant material is deposited in the fractures, the hardenable resin composition hardens and consolidates the proppant material into one or more permeable packs having high compressive strengths.

[0019] As mentioned, when a viscous aqueous fracturing fluid or a viscous portion of an aqueous fracturing fluid is utilized, it can include a delayed breaker for effecting a controlled reduction in the viscosity of the aqueous fracturing fluid so that the proppant material is deposited in the fractures and the fracturing fluid is recovered. Examples of delayed breakers which can be utilized include, but are not limited to, sodium perborate, potassium periodate, sodium persulfate, t-butyl hydroperoxide, sodium bromate, lithium hypochlorite, sodium hypochlorite, and sodium chlorite. Of these, sodium perborate is preferred. When used, the delayed breaker is included in the aqueous fracturing fluid in an amount in the range of from-about 0.01% to about 5% by weight of gelling agent in the aqueous fracturing fluid, more preferably in an amount of about 1%.

[0020] A preferred method of this invention for fracturing a subterranean zone penetrated by a well bore comprises the steps of: (a) providing an aqueous fracturing fluid comprising a brine having a density in the range of from about 9 to about 19 pounds per gallon; (b) pumping the aqueous fracturing fluid into the subterranean zone at a rate and pressure sufficient to fracture the subterranean zone; and (c) recovering the aqueous fracturing fluid from the subterranean zone.

[0021] Another preferred method of this invention for fracturing a subterranean zone penetrated by a well bore comprises the steps of: (a) providing a viscous aqueous fracturing fluid comprising a brine having a density in the range of from about 9 to about 19 pounds per gallon and a gelling agent; (b) pumping the viscous aqueous fracturing fluid into the subterranean zone at a rate and pressure sufficient to fracture the subterranean zone; and (c) recovering the viscous aqueous fracturing fluid from the subterranean zone.

[0022] Still another preferred method of this invention for fracturing a subterranean zone penetrated by a well bore comprises the steps of: (a) providing a viscous aqueous fracturing fluid comprising calcium chloride brine having a density in the range of from about 9 to about 19 pounds per gallon, a xanthan gelling agent present in the viscous aqueous fracturing fluid in an amount in the range of from about 0.1% to 2% by weight of the brine therein, a graded sand proppant material present in the viscous aqueous fracturing fluid in an amount in the range of from about 1 pound to about 18 pounds per gallon of the viscous aqueous fracturing fluid and a sodium perborate delayed breaker present in the viscous aqueous fracturing fluid in an amount in the range of from about 0.01% to about 5% by weight of gelling agent therein; (b) pumping the viscous aqueous fracturing fluid into the subterranean zone at a rate and pressure sufficient to fracture the subterranean zone; and (c) recovering the aqueous fracturing fluid from the subterranean zone.

[0023] In order to further illustrate the methods and aqueous treating fluids of the present invention, the following example is given.

EXAMPLE 1

[0024] Wellhead pressures were calculated for water (8.33 lb/gal density and 0.9 cP viscosity) and brine (12 lb/gal density and 4 cP viscosity) at various flow rates through 3″ ID and 4″ ID pipe strings disposed in a 10,000 ft well having a fracture gradient of 0.75 psi/ft. The wellhead pressures for the water and brine were calculated for pure water and brine, for water and brine including succinoglycan polymer at a concentration of 35 lb/1000 gal and for water and brine including xanthan polymer at a concentration of 35 lb/1000 gal. The results of the calculations are set forth in Table 1 below. TABLE 1 Wellhead Pressure Calculations Rate Well Head Pressure, psi bbls/ Pipe ID, No Polymer Succinoglycan Xanthan min inches Water Brine Water Brine Water Brine 1 3 3194 1305 3265 1359 3297 1391 5 3 3573 1960 3336 1549 3400 1660 10 3 4650 3732 3813 2335 3723 2185 20 3 8761 10225 5670 5270 4862 3974 30 3 15464 20535 8724 9968 6636 6696 1 4 3178 1276 3230 1323 3245 1338 5 4 3270 1439 3255 1348 3290 1451 10 4 3525 1872 3310 1502 3340 1554 20 4 4482 3432 3774 2260 3587 1952 30 4 6024 5875 4556 3502 3960 2539

EXAMPLE 2

[0025] Example 1 was repeated except that the brine had a density of 15 lb/gal and a viscosity of 10 cP. The results of the calculations are set forth in Table 2 below. TABLE 2 Wellhead Pressure Calculations Rate, Pipe Well Head Pressure, psi bbls/ ID, in- No Polymer Succinoglycan Xanthan min ches Water Brine Water Brine Water Brine 1 3 3194 −234 3265 −200 3297 −168 5 3 3573 702 3336 113 3400 272 10 3 4650 3176 3813 1211 3723 1001 20 3 8761 12036 5670 5218 4862 3434 30 3 15464 25855 8724 11516 6636 7076 1 4 3178 −277 3230 −235 3245 −220 5 4 3270 −42 3255 −210 3290 −24 10 4 3525 571 3310 43 3340 118 20 4 4482 2737 3774 1098 3587 666 30 4 6024 6074 4556 2797 3960 1467

[0026] From the Tables, it can be seen that the well head pressure of brine is lower than the well head pressure of water at rates up to and including 20 bbls/min. Also, it can be seen that the well head pressure of brine containing succinoglycan or xanthan polymers is lower than the well head pressure of water at rates up to and including 30 bbls/min.

[0027] Thus, the present invention is well adapted to carry out the objects and attain the ends and advantages mentioned as well as those which are inherent therein. While numerous changes can be made by those skilled in the art, such changes are encompassed within the spirit of this invention as defined by the appended claims.

What is claimed is: 

1. A method of fracturing a subterranean zone penetrated by a well bore comprising the steps of: (a) providing an aqueous fracturing fluid that comprises a brine having a density in the range of from about 9 to 19 pounds per gallon; (b) pumping said aqueous fracturing fluid into said subterranean zone at a rate and pressure sufficient to fracture said subterranean zone; and (c) recovering said aqueous fracturing fluid from said subterranean zone.
 2. The method of claim 1 wherein said brine is selected from the group consisting of calcium chloride brine, calcium bromide brine, sodium bromide brine, sodium chloride brine, potassium formate brine, cesium formate brine, zinc chloride brine, zinc bromide brine, and mixtures thereof.
 3. The method of claim 1 wherein said brine is calcium chloride brine.
 4. A method of fracturing a subterranean zone penetrated by a well bore comprising the steps of: (a) providing a viscous aqueous fracturing fluid that comprises a brine having a density in the range of from about 9 to about 19 pounds per gallon and a gelling agent; (b) pumping said viscous aqueous fracturing fluid into said subterranean zone at a rate and pressure sufficient to fracture said subterranean zone; and (c) recovering said aqueous fracturing fluid from said subterranean zone.
 5. The method of claim 4 wherein said brine is selected from the group consisting of calcium chloride brine, calcium bromide brine, sodium bromide brine, sodium chloride brine, potassium formate brine, cesium formate brine, zinc chloride brine, zinc bromide brine, and mixtures thereof.
 6. The method of claim 4 wherein said brine is calcium chloride brine.
 7. The method of claim 4 wherein said gelling agent is selected from the group consisting of hydroxyethylcellulose, guar, xanthan and succinoglycan.
 8. The method of claim 4 wherein said gelling agent is xanthan.
 9. The method of claim 4 wherein said gelling agent is present in said aqueous fracturing fluid in an amount in the range of from about 0.1% to about 2% by weight of said brine therein.
 10. The method of claim 4 wherein said aqueous fracturing fluid further comprises a cross-linking agent for cross-linking said gelling agent.
 11. The method of claim 10 wherein said cross-linking agent is selected from the group consisting of sodium borate decahydrate, zirconium oxychloride, calcium salts, aluminum salts, magnesium salts, iron compounds, iodine compounds and baron compounds.
 12. The method of claim 10 wherein said cross-linking agent is sodium borate decahydrate.
 13. The method of claim 10 wherein said cross-linking agent is present in said aqueous fracturing fluid in an amount in the range of from about 0.1% to about 66% by weight of said gelling agent therein.
 14. The method of claim 4 wherein said aqueous fracturing fluid further comprises a proppant material.
 15. The method of claim 14 wherein said proppant material is selected from the group consisting of graded sand, sintered bauxite, walnut hulls, and glass beads.
 16. The method of claim 14 wherein said proppant material is graded sand.
 17. The method of claim 14 wherein at least part of said proppant material is coated with a hardenable resin composition.
 18. The method of claim 14 wherein said proppant material is present in said aqueous fracturing fluid in an amount in the range of from about 1 to about 18 pounds per gallon of said fracturing fluid.
 19. The method of claim 4 wherein said aqueous fracturing fluid further comprises a delayed breaker for effecting a controlled reduction in viscosity of said aqueous fracturing fluid.
 20. The method of claim 19 wherein said delayed breaker is selected from the group consisting of sodium perborate, potassium periodate, sodium persulfate, t-butyl hydroperoxide, sodium bromate, lithium hypochlorite, sodium hypochlorite and sodium chlorite.
 21. The method of claim 19 wherein said delayed breaker is sodium perborate.
 22. The method of claim 19 wherein said delayed breaker is present in said aqueous fracturing fluid in an amount in the range of from about 0.01% to about 5% by weight of gelling agent therein.
 23. A method of fracturing a subterranean zone penetrated by a well bore comprising the steps of: (a) providing a viscous aqueous fracturing fluid that comprises calcium chloride brine having a density in the range of from about 9 to about 19 pounds per gallon, a xanthan gelling agent present in said viscous fracturing fluid in an amount in the range of from about 0.1% to about 2% by weight of said brine therein, a graded sand proppant material present in said viscous fracturing fluid in an amount in the range of from about 1 pound to about 18 pounds per gallon of said viscous fracturing fluid, and a sodium perborate delayed breaker present in said viscous aqueous fracturing fluid in an amount in the range of from about 0.01% to about 5% by weight of said gelling agent therein; (b) pumping said viscous aqueous fracturing fluid into said subterranean zone at a rate and pressure sufficient to fracture said subterranean zone; and (c) recovering said aqueous fracturing fluid from said subterranean zone.
 24. The method of claim 23 wherein said aqueous fracturing fluid further comprises a cross-linking agent comprising sodium perborate tetrahydrate present in said aqueous fracturing fluid in an amount in the range of from about 0.01% to about 5% by weight of said gelling agent therein. 